1. Field of the Invention
This invention relates to turbomachinery and is particularly directed to turbomachinery having ceramic shields as thermal protection for blades and vanes for high-temperature operation.
2. Description of the Prior Art
In order to improve the performance and fuel economy of turbomachinery, such as pumps or turbines, it has been proposed to operate the turbines at elevated turbine inlet temperatures. Inlet temperatures above 2400.degree. F. are theoretically desirable. However, such temperatures are well above the operating capabilities of even the most advanced high-strength metals unless complex and costly cooling methods are applied to the blades' exterior surfaces.
Blades comprising high-temperature ceramics have exhibited great potential for fulfilling the goal of accommodating high turbine inlet temperatures without requiring the use of complex surface cooling methods. However, since ceramics are brittle and have little capacity for withstanding mechanical or thermally induced tensile stresses, various significant problems arise in connection with the application of ceramics to turbine blade and stator vane design.
A typical example of a ceramic turbine blade constructed according to the prior art can be found in U.S. Pat. No. 2,749,057 to Bodger which discloses a turbine rotor having a row of blades, each blade comprising a central post integral with the rotor and a hollowed ceramic blade element of airfoil shape mounted onto the post. A cap member is affixed to the outer tip of the post which serves as an abutment to the ceramic shield against centrifugal movement. During rotation, the ceramic blade element bears against and is supported by the cap member so that tensile loading of the ceramic blade element is avoided. Among the other features disclosed in Bodger include a central cooling duct through the post for effecting cooling of the post's exterior surfaces.
Because it is the exterior surface of the post where cooling is most needed, it has been found that the use of a central cooling duct as taught in Bodger requires prohibitively large volumes of cooling air in order to be effective. An alternative arrangement in the prior art attempts to avoid this shortcoming by directing cooling air through a gap between the ceramic blade element and the post, as exemplified by the device shown in FIG. 1 of French Pat. No. 57,426 to Bolsezian, wherein cooling air is ducted directly from the rotor hub. However, such device requires that the ceramic blade element be protected from the passing cooling fluid so that a destructive thermal gradient is not built-up in the ceramic blade element. Bolsezian attempts to accomplish this by coverng the interior surfaces of the ceramic blade element with a layer of thermally insulating material; however, such construction requires the difficult and costly step of bonding a layer of insulation directly to the interior ceramic blade element and makes the whole blade assembly vulnerable to failure upon breach of the insulatory layer, however slight the breach.
Another significant disadvantage of a blade constructed according to the prior art is that the ceramic blade element is restrained only at its footing and tip without means for dampening vibration or relieving aerodynamically induced stresses along the entire surfaces of the blade. For instance, in Bodger a rim at the tip of the ceramic blade element is provided for bearing against the cap in a manner resistive to the ceramic blade elements tendency to rotate when aerodynamically loaded. Such arrangment not only aggravates the risk of blade failure by subjecting the tip of ceramic blade element to localized, mechanical stresses but also fails to provide means for resisting such angular displacement uniformly across the whole span of the blade.
Additionally, since the ceramic blade element in either the Bodger-type or the Bosezian-type blade is supported only at its ends and is in close proximity to the post member, any transient or nodal vibration in either element might lead to one bearing against the other in a manner destructive of the ceramic blade element. This disadvantage is especially critical in blades constructed according to Bolsezian where the vibrationally-induced contact might breach the layer of insulation.
Of the more troublesome sources of vibration is the flutter created each time a turbine blade traverses in proximity to one of turbine inlet vanes comprising the turbine stage. Because each vane acts somewhat like a baffle, each turbine blade is subjected to a high rate of cyclical variation in aerodynamic loading as each blade proceeds from one of the more baffled regions of flow to one of the less baffled regions of flow and back again. These circumstances present significant problems to one constructing a viable ceramic blade element because cyclical fatigue is a principal mode of failure for ceramic materials. Unless means are taken to dampen this cyclical flutter failure is likely to occur.
Practitioners of the prior art seem intent on overcoming these problems by thickening the walls of the ceramic blade elements so that they are more resistive to the vibration induced stresses. However this solution creates further problems of its own in that, as a ceramic element is made thicker, it is caused to carry a greater and greater thermal gradient across its thickness, which, in the realm of turbine flow temperatures, can lead to the creation of destructive levels of internal stresses. So it appears that the thickening of the ceramic blade element is a disadvantageous means for overcoming the problems associated with vibration and a more effective alternative is most desired.